This invention relates to the preparation of highly densified ceramic bodies by the pyrolysis of certain preceramic polysilazanes filled with silicon carbide powders, certain metal-containing sintering aids, and, optionally, certain polysilazane curing agents. Such highly densified ceramic bodies can be prepared by either a pressureless sintering process or a hot press sintering process. This invention also relates to novel compositions comprising silicon carbide powder, a metal-containing sintering aid, a preceramic polysilazane, and, optionally, a polysilazane curing agent. These compositions of matter can be formed into green bodies of the desired shape and sintered to form ceramic, shaped bodies with high densities. One advantage of the present invention is that the green bodies have relative high strengths and thus can be easily handled and, if desired, machined before sintering. Green bodies prepared from silacyclobutasilazane polymers or polysilazanes containing polysilazane curing agents may be cured prior to the sintering step thereby increasing the strength.
Prockazka in U.S. Pat. Nos. 4,004,934 and 4,041,117 and Prockazka et al. in U.S. Pat. No. 4,081,284 describe high density silicon carbide ceramic bodies prepared in a pressureless sintering process. Such ceramic bodies were prepared from mixtures consisting of silicon carbide powder, a boron-containing additive, and a carbonaceous additive. The carbonaceous additives or binders were high molecular weight, carbon-based aromatic compounds such as phenol-formaldehyde condensate-novolak, resorcinol-formaldehyde, aniline-formaldehyde, cresol-formaldehyde, derivatives of polynuclear aromatic hydrocarbon compounds contained in coal tar, polyphenylene, polymethylphenylene, and the like. Ceramic silicon carbide materials with greater than 85% of theoretical density were obtained.
Onda et al. in Japanese Kokai Patent No. 60-16869 describe a method of preparing high density silicon carbide ceramics which involves pressureless sintering a mixture consisting of silicon carbide powder, an auxiliary sintering aid, and an organosilicon polymer characterized by a Si-C skeletal structure. Disclosed sintering aids include boron-, aluminum-, and berylliumcontaining compounds. The organosilicon polymers were described as "high-molecular-weight organic silicon-containing compounds with siliconcarbon skeletal structures." The organosilicon polymers were not further identified even in the examples. Based on this limited description, however, it appears that the organosilicon polymers were polycarbosilanes which have a Si-C skeletal backbone.
The present invention provides high density ceramic products from a pressureless sintering process or a hot press sintering process using polysilazanes as binders. Generally, high density ceramic materials will possess high strengths. One unique aspect of this invention is that a Si-N containing material is used as a precursor for a silicon carbide material which contains only limited amounts of nitrogen.